Electrorefining of elemental boron



June 4, 1960 A. A. UCHIYAMA ETAL 2,940,911

' ELECTROREFINING OF ELEMENTAL BORON Filed Jan. 2. 1959 Iner 7 gas sweep Cafhod: Ouf/e 7 7 Incone/ canfa/ner (Posifive) If Crude Baron 6 Graphif''.

- 12 Perforated cmho d e Graph/fa (Negafl ve) Cylinder f/ Crude Boron Perfofafcd Graph/fa Cylinder 1N VE N 7'0 R5 A MEMBER OF THE ELECTROREFINING F ELEMENTAL BORON Aiji Alvin Uchiyama, Pasadena, David Russell Stern, Fullei-ton, and Quentin Hyde McKeuna, Whittier, Califi, assignors to American Potash & Chemical Corporation, a corporation of Delaware Filed Jan. 2, 1959, Ser. No. 784,475

8 Claims. (Cl. 204-60) To obtain boron of high purity, a subsequent operation such as degassing or chlorination is required to upgrade the product. For example, boron produced by the thermal reduction of boric oxide with magnesium metal usually contains from 4-14% magnesium as an impurity. This impurity does not exist as free leachable magnesium metal but appears as an unleachable entity trapped in the boron lattice. This boron product may be satisfactory for a rocket igniter but may not be suitable as an atomic reactor shield because of undesirable side reactions of the contaminants. This type of boron has subsequently been upgraded by vacuum degassing or chlorination technique.

In general, this invention involves the preparation of a purified boron from a relatively crude boron in a fused salt electrolytic system. We have found that if a crude boron is made an anode and submerged in a fused salt bath containing an alkali chloride or a mixture of alkali chlorides and potassium fiuoborate, the boron contained in the crude boron passes into the solution leaving a residue of the impurities. The dissolved boron under the intluence of a direct current is transported to the cathode where it is deposited as elemental boron. In essence, a purified boron is made from a crude boron employed as a consumable anode by what we consider to be an anodic transfer with no appreciable electrolysis of the fiuoborate or the alkali chlorides.

The process is best performed at a potential which is too low (i.e., less than 3 volts and preferably below 2 volts) to reduce electrolytically the potassium fluoborate contained in the bath, and to discharge chlorine gas at the anode. Since very low potentials are utilized in this process (considerably less than conventional electrolytic processes), a low energy requirement per unit weight of boron is realized, a cathode current density sufficing which is usually from 0.5 to 4.0 amps. per square inch. Moreover, since low temperatures of operation are possible, higher current efiiciencies as well as greater ease of operation are realized. The temperature of operation can be in the range of about 350 to 1000 C.

We have found that the presence of from 3% to 50% of the double fluoride salt of boron is essential and is apparently involved in the anodic dissolution of boron in the crude boron anode. We now believe that the BF; ion enters into an oxidation-reduction reaction with boron, producing a new ionic species which is subsequently reduced at the cathode to elemental boron. The BF; is regenerated by being reoxidized, probably at the anode, and is again available for the reaction With the boron anode. Since the concentration of fluoborate in the bath is essentially not reduced under the electrolysis conditions described here, it appears that it acts as a carrier of the boron supplied by the boron anode, but we do not wish to be limited by this theory of operation.

nited States Patent 0 2,940,911 Patented June 14, 1960 Other double fluoride salts may also be employed such as NaBF, and LiBF An electrolytic cell for use in practicing this invention is shown in Figure 1. This includes a graphite or carbon crucible 6 placed inside a heat resistant metal shell 7 made of Inconel or other suitable metal. The outer Inconel pot is fitted with an Inconel lid 8 and an asbestos gasket 9 so that the melt can be kept under an inert atmosphere during electrolysis. The cell can be externally heated or internally heated by electrical resistance heating in the bath itself. The graphite crucible is made anodic by connecting the metal shell to the positive terminal of the direct current source. The boron is made anodic by packing it, as at 11, as a crude material between the inner crucible 6 and a perforated carbon cylinder 12 which acts as a mechanical support for the boron. The boron can be in the form of a powder, granules or rods; in fact, in any form. It is obvious to those skilled in the art that this container can be designed such that the residual crude boron remaining after electrolysis can be removed and a new boron anode inserted. The cathode, which can be an iron or a mild steel bar 14, is suspended vertically through the lid into the bath inside the carbon barrier.

EXAMPLE 1 The following example is illustrative practice of the investigation. A mixture of reagent gradeNaCl and KCl consisting of 40 weight percent KCl and 40 weight percent NaCl was charged into a graphite crucible which had been packed with a commercial grade of amorphous boron between the crucible wall and the perforated graphite retaining cylinder. The charged crucible was placed in an Inconel pot, the lid fastened in place, and the entire assembly set in a furnace and heated to about 800 C. to melt the alkali chlorides under an inert gas (argon) sweep. Next, potassium fiuoborate (KBF was added in an amount that gave a final 20 weight percent concentration of KBF The bath temperature was then adjusted to the desired operating temperature which in this case can range between about 500 and 1000 C. However, We prefer to operate at about 800 C. A mild steel rodshaped cathode was introduced to the melt through a port provided in the shell lid and the electrolysis performed under argon gas sweep.

The voltage was held at 1.1 volts and the current 'at 3.5 amps. or 1.36 amp/sq. in. As the boron was deposited on the cathode, no evolution of chlorine occurred at the anode surface. During electrolysis, the current fiow remained constant and tended to increase slightly with time. No increase in voltage was noted with time, as the cathode deposit increased, which is not true of other electrolytic boron processes. The current efficiency was 91%. a

At the completion of the run the current was turned off and the cathode raised out of the bath but kept within the. cell to cool in an argon atmosphere. Although in these experiments argon was used as an inert gas sweep, we have found that the deposits may be removed relatively hot into the air for the electrolyte which coats the boron deposit acts "as a protective coating and prevents undue oxidation. After cooling, the boron deposit, coated with occluded salt, was readily broken away from the cathode and washed thoroughly with water, then with concentrated hydrochloric acid. Finally, it was rinsed with water and then acetone and dried in a vacuum oven at 35-45 C.

The product is in the form of small amorphous particles held together as spongy appearing aggregates. The

- not'more than "about 50%. A' satisfactory electrolysis following analyses show how the purity of the product I was improved from a commercial grade of boron;

' TABLE 1 1 5 Initial Boron Final Product 7 roan 13:84.54 B=94.7s

Mg= 5.61 Mg: 1.41 V

Fe= 0.09 Fe= 0.0a

Upon completion of the electrolysis and removal of the cathode, another-cathode can be introduced immediv ately and a second electrolysis performed. These cycles can be repeated until' the available boron in the crude boron source has been reduced to about 20% of its original content, which is manifested in a reduction of the current flow at, comparable voltages. The residue can then be removed and fresh cmde boron added.

' If other electrolyte mixtures areused such as potassium chloride and lithium'chloride, the'temperature of operation can be lowered to about 375 C.jto 650 C. However we find that the best temperature of operation lies/in the range of 750 to 850? C. The fluoborate should not be less than 3% by weight-of the mixture and cannot be performed'in a l00%'fluoborate bath due to the extreme difiiculty in reaching a sufficiently high a cathode current: density to overcome the bath reaction between'boron depositing on the cathode and the BF; ionsinthe'melt. a a V The alkali chloride reduces the BF; ion concentraa tion so that satisfactory cathode current densities can be obtaiined'where the rate of deposition of boron is much greater than the rate of resolution of the deposited boron. Also, the presence of the chloride lowers the temperature of operation. Asa chloride providing material one can use KCl, N'aCL LiCl, CaCl and mixtures of these. 'One can also use lithium or sodiumfluoborate.

Further illustrative runs are as follows: 7 7 4o EXAMPLE 2 a I Sodium contaminated boron Electrolyte 'composition NaCl, 40 wt. percent; KCl,

1' j V v 40'wt'.percent;'KBF ,20 V V wtpercent.

,Yoltage; 1.0 volts. V Current 1.8"iIII1PS. Cathode current density"; 0.8 amp./sq.in. V V Temperature -800 C. r a "Current efficiency 99% I- l nitial Boron Final Bor on 3:91.97. I B= 97.39 V V Na='3.48 Na: 0.18 w Fe: 0.17 V- Fe= 0.12 rtNo,Inso1.= 2190 HNO 1uso1.= 0.36

'EXAMPLE3 Carbon contaminated boron 40 wt percent; KBF 20 wt. percent ".Voltage a 1.0 volts. 7 7

Current 6.9 amps t Cathode current density. 0.9 amp./sq.in.'

Temperature 800 C. Current efliciency 98% Initial Final Boron Boron Weclaimz I 1 a 1. A process for. production of apurified boron comprising passinga current between a crude boron anode and a cathode undera potentialof less than about 3 volts, through a fused salt bath at a temperature between about 350 C. an'd about 1000 C. to deposit boron on the cathode, the bath" consisting essentially of an alkali metal chloride and an-alkali metal ,fluoborate, the fluoborate providing from about 3% to about 50% of the mixture.

'2. A process as in claim 1 wherein the anode is provided by granular boron. 7

31A process as in claim 1 wherein the anode is provided by amorphous boron.

4. A process as in claim 1 wherem the anode is provided by crystalline boron. '5. A process for production of a purified boron eomprising passing a current between a boron anode and a cathode under a potential of less than about 3 volts through a fused salt bath at a temperature between about 1 350 C. and about 1000 C. to deposit boron on the cathode, the bath consisting essentially of an alkali chloride and potassium fluoborateythefiuoborate providing from about 3% to about 50% of the mixture.

6.- A process for production of a purified boron comprising passing a current betweena boron anode and a cathode under .a potential of less than about '3 "volts through a fused salt bath at a temperature between about 350 C. and about 1000 C. to deposit boron on the cathode, the bath consisting essentially of a mixture of V alkali chlorides and potassium fluoborate, the fluoborate providing fromtabout 3% to about 50% of the mixture.

7; A process for production of a purified boron comprising passing a current between a boron anode and a cathode under a potenn'alof less than about}: volts through a 'fused salt bath at a temperature between about 7 350 C. and about 1000 .,C. to deposit boron on the cathodethebath consisting essentially of analkali chlo- I ride and? sodium fiuoborate, the fiuoborate providing from about 3% to about 50%of the mixture. 77

'8. A procpes s for production of a'purified boron' comprisin'g passinga current between a boron anode and a cathode under a'potential of less than about 3"volts through a fused salt bath'at a temperature between about 350 .C. and about 1000" C to deposit boron on the cathode, the bath consisting essentially of an alkali chloride and lithium fluoborate, the fluoborate providing from about 3% to about 50% of the mixture. 1

iReferenees Cited in the file of this patent n UNITED STATES PATENTS 2, s72;24s Cooper j Oct. 23, 1951 l 2,832,730 Nies et a1. V. V V Apr. 29,1958

FOREIGN PATENTS 164,170 Australia 'AprL8,1955

. 6 was M1 H 

1. A PROCESS FOR PRODUCTION OF A PURIFIED BORON COMPRISING PASSING A CURRENT BETWEEN A CRUDE BORON ANODE AND A CATHODE UNDER A POTENTIAL OF LESS THAN ABOUT 3 VOLTS, THROUGH A FUSED SALT BATH AT A TEMPERATURE BETWEEN ABOUT 350*C. AND ABOUT 1000*C. TO DEPOSIT BORON ON THE CATHODE, THE BATH CONSISTING ESSENTIALLY OF AN ALKALI METAL CHLORIDE AND AN ALKALI METAL FLUOBORATE, THE FLUOBORATE PROVIDING FROM ABOUT 3% TO ABOUT 50% OF THE MIXTURE. 